Krathwohl, W. C. (1951). Relative contributions of vocabulary and an index of industriousness for English to achievement in English. Journal of Educational Psychology, 42(2), 97-104.
http://dx.doi.org/10.1037/h0061720
An index of industriousness for English can be based on the difference between an individual's aptitude and achievement scores, when both scores have been reduced to comparable standard units. Indices of this type and vocabulary scores were determined for a group of 308 students and the relationships of these to English achievement were analyzed. It is concluded that work habits and vocabulary size are the two major determiners of English achievement. (PsycINFO Database Record (c) 2016 APA, all rights reserved)